Setup:

The problem is with leftmost assembler. Their output is limited by production of green circuts -> inserter -> belt. Used express belts. Theoretically: Production with these modules (ignore green) is 22,5 green circuts per second and half od express belt is the same, 22,5 items per second. But I tested and output is slighty lower. I test different inserters capacity, nothing change, but one assembler produce 7,5 item per second so, mass inserter can move more items than assembler produce per second- from wiki, 13,85 items per second. Even add time to move arm, inserter to belt is more.
So, what is a problem? Belt nature? Is a problem with 4th dimmension?
P.S Is not a problem with output out of the setup.